JAKARTA (Reuters) - Three volcanoes in Indonesia, including the one known as the "Child of Krakatau," are now under close watch following heightened activity, a senior volcano official said on Saturday.

Indonesia's Centre for Volcanology and Geological Hazard Mitigation raised the alert on Mount Anak Krakatau to the second-highest level on Friday after it threw up showers of ash.

The volcano, which lies in the Sunda strait between the islands of Java and Sumatra, is about 130 km west of the capital Jakarta. It gradually formed after the famous Krakatau volcano blew up in a massive eruption in 1883, triggering tsunamis and killing thousands of people.

Saut Simatupang, a senior official at the centre, said that volcanic tremors at Anak Krakatau, which is a popular tourist attraction, had increased in the past two days.

Officials are also monitoring two other volcanoes. Mount Kelud volcano in East Java has been on the highest state of alert for several days as it appears to be very close to erupting.

The volcano, which has a lake in its crater, is about 90 km southwest of Surabaya, Indonesia's second-biggest city.

Mount Soputan, in North Sulawesi, erupted last week spewing columns of ash 1,000 meters (3,300 ft) into the air, but its activity has since decreased, Simatupang said.

Indonesia has the highest number of active volcanoes in any country, sitting on a belt of intense volcanic and seismic activity known as the "Pacific Ring of Fire."